Enhanced channel parameter estimation in the presence of preamble erasures

ABSTRACT

A communication device processes a preamble in the presence of impulse noise. The communication device receives a preamble sequence that includes a plurality of preamble symbols and determines that at least one preamble symbol of the plurality of preamble symbols has been adversely affected by impulse noise. Based upon this determination, the communication device masks the at least one preamble symbol of the plurality of preamble symbols to produce a subgroup of preamble symbols. Then, based upon the subgroup of preamble symbols, the communication device determines at least one of a frequency estimate for the subgroup of preamble symbols, a phase estimate for the subgroup of preamble symbols, and a gain estimate for the subgroup of preamble symbols. Based upon these estimates, the communication receiver corrects subgroup of preamble symbols and uses the result to perform channel estimation, to correct subsequently received symbols carrying data, etc., and performs additional operations based upon the corrected preamble symbols.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional ApplicationSerial No. 60/416,928, filed Oct. 8, 2002, which is incorporated hereinby reference in its entirety for all purposes.

BACKGROUND

[0002] 1. Field of the Invention

[0003] This invention relates generally to digital communications; andmore particularly to preamble processing in a digital communicationsystem.

[0004] 2. Background of the Invention

[0005] The structure and operation of communication systems is generallyknown. Many communication systems carry data, e.g., voice, audio, video,file, or other digital data that is sent from a transmitter to areceiver. On the transmitter side, data is first formed into packets.This data may be raw data or encoded data that represents the raw data.Each of these packets also typically includes a header, a known trainingsequence generally referred to as the “preamble”, and a tail. Thesepackets are then modulated into symbols and the symbols are transmittedby the receiver and intended for receipt by the receiver. The receiverthen receives the symbols and attempt to extract the data from thepackets that are carried by the symbols.

[0006] A “channel” carries the symbols from the transmitter to thereceiver. A wired, wireless, optical, or another media, depending uponthe communication system type, services the channel. In manycommunication systems, such as terrestrial based wireless communicationsystems, satellite based communication systems, cable basedcommunication systems, etc., the channel distorts the transmittedsymbols, from the perspective of the receiver, causing interferencebetween a subject symbol and a plurality of symbols surrounding thesubject symbol. This type of distortion is referred to as“inter-symbol-interference” and is, generally speaking, thetime-dispersed receipt of multiple copies the symbols caused bymultipath. The channel also introduces noise, e.g., impulse (burst)noise, into the symbols prior to their receipt. Each of these conceptsis well known.

[0007] In many communications systems, a preamble sequence is used toestimate channel parameters such as carrier frequency and phase offsets,and channel gain. The channel estimate may also be used for equalizertraining. In many cases, the received preamble symbols are distorted bythe impulse (burst) noise. Although the burst noise might distort alimited number of preamble symbols, it might be strong enough such thatwhole preamble processing is degraded, which causes the communicationsystem to loose lock (or synchronization). Degradation of preambleprocessing may lead to system crashes in the case of continuoustransmission systems, and packet losses and reduction in the overallsystem capacity for packet transmission systems.

[0008] Thus there is a need in the art for improved preamble processing.

SUMMARY OF THE INVENTION

[0009] In order to overcome the shortcomings of the prior devices, amongother shortcomings, a communication device constructed according to thepresent invention processes a preamble in the presence of impulse noise.In its operations, the communication device receives a preamble sequencethat includes a plurality of preamble symbols. The communication devicethen determines that at least one preamble symbol of the plurality ofpreamble symbols has been adversely affected by impulse noise. Basedupon this determination, the communication device masks the at least onepreamble symbol of the plurality of preamble symbols to produce asubgroup of preamble symbols. Then, based upon the subgroup of preamblesymbols, the communication device determines at least one of a frequencyestimate for the subgroup of preamble symbols, a phase estimate for thesubgroup of preamble symbols, and a gain estimate for the subgroup ofpreamble symbols.

[0010] In determining that at least one preamble symbol of the pluralityof preamble symbols has been adversely affected by impulse noise, thecommunication device divides the plurality of preamble symbols by atleast one known preamble symbol to produce a plurality of preamble gainscorresponding to the plurality of preamble symbols. Finally, thecommunication device determines, based upon the plurality of preamblegains, that at least one preamble symbol has been adversely affected byimpulse noise. In determining, based upon the plurality of preamblegains, that at least one preamble symbol has been adversely affected byimpulse noise, the communication receiver first determines a gaindifferential sequence from the plurality of preamble gains that includesa plurality of gain differential values. The communication device then,for each gain differential value that exceeds a gain differentialthreshold, determines that a corresponding preamble symbol has beenadversely affected by impulse noise.

[0011] In another operation, in dividing the plurality of preamblesymbols by at least one known preamble symbol further produces aplurality of preamble phases corresponding to the plurality of preamblesymbols. In such case, the communication device also determines, basedupon the plurality of preamble phases, that at least one preamble symbolhas been adversely affected by impulse noise. In one particularoperation, the communication device determines from the plurality ofpreamble phases a phase differential sequence that includes a pluralityof phase differential values. Then, for each phase differential valuethat exceeds a phase differential threshold, the communication devicedetermines that a corresponding preamble symbol has been adverselyaffected by impulse noise.

[0012] In its further operations, the communication device masks(discards) at least one preamble symbol that has been adversely affectedby impulse noise from the plurality of preamble symbols. In such case,the communication device may combine non-discarded preamble symbols ofthe plurality of preamble symbols of the preamble sequence to produce acomposite result. Then, the communication device may apply at least onecorrection factor to the non-discarded preamble symbols of the pluralityof preamble symbols of the preamble sequence to produce the compositeresult.

[0013] The communication device may service either wired or wirelesslinks. In servicing wired links, the communication device may be a cablemodem communication system receiver. In such case, the cable modemcommunication system is either a Cable Modem Termination System or acable modem.

[0014] Other features and advantages of the present invention willbecome apparent from the following detailed description of the inventionmade with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] A better understanding of the invention can be obtained when thefollowing detailed description of various exemplary embodiments isconsidered in conjunction with the following drawings.

[0016]FIG. 1 is a system diagram illustrating a Cable Modem (CM)communication system that operates according to the invention;

[0017]FIG. 2 is a system diagram illustrating in more detail a CableModem Termination System (CMTS) and a CM of the present invention;

[0018]FIG. 3 is a system diagram illustrating a cellular communicationsystem having a base station receiver that services a reverse linkaccording the invention;

[0019]FIG. 4 is a system diagram illustrating a cellular communicationsystem having a mobile receiver that services a forward link accordingthe invention;

[0020]FIG. 5 is a system diagram illustrating a satellite communicationsystem and a fixed wireless communication system, each operatingaccording to the invention;

[0021]FIG. 6 is a block diagram illustrating a CMTS built according tothe present invention;

[0022]FIG. 7 is a graph illustrating preamble and data symbols receivedand operated upon by a CMTS (or another) receiver according to thepresent invention;

[0023]FIG. 8 is a logic diagram illustrating operation of the presentinvention in operating upon a preamble to detect burst (impulse) noiseand in operating upon the preamble based upon the determination;

[0024]FIG. 9 is a graph illustrating preamble gain of a plurality ofpreamble symbols as determined according to the present invention;

[0025]FIG. 10 is a graph illustrating delta preamble gain of a pluralityof preamble symbols and related error detection as determined accordingto the present invention;

[0026]FIG. 11 is a graph illustrating preamble phase of a plurality ofpreamble symbols as determined according to the present invention;

[0027]FIG. 12 is a graph illustrating delta preamble phase of aplurality of preamble symbols and related error detection as determinedaccording to the present invention;

[0028]FIG. 13 is a graph illustrating erasure flags corresponding to aplurality of preamble symbols as determined according to the presentinvention;

[0029]FIG. 14 is a logic diagram illustrating operation of the presentinvention in operating upon a preamble to detect burst (impulse) noise,to erase preamble symbols, to combine good preamble symbols, and todetermine frequency, phase, and gain estimates of the preamble;

[0030]FIG. 15 is a graph illustrating how system performance improveswhen operating according to the present invention; and

[0031]FIG. 16 is a block diagram illustrating a communication deviceconstructed according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0032]FIG. 1 is a system diagram illustrating a Cable Modem (CM)communication system 100 that operates according to the invention. TheCM communication system 100 includes a number of Cable Modems (CMs), CM1 111, CM 2 115, . . . , and CM n 121, a Cable Headend Transmitter 120,and a Cable Modem Termination System (CMTS) 130A or 130B. The CMTS 130Aor 130B is a component that exchanges modulated digital information withCMs across a cable network segment 199. A number of elements may beincluded within the CM network segment 199. For example, routers,splitters, couplers, relays, and amplifiers may be contained within theCM network segment 199 without departing from the scope and spirit ofthe invention.

[0033] In some embodiments, the CMTS 130A is contained within the CableHeadend Transmitter 120. In other embodiments, a CMTS 130B is locatedexternally with respect to the Cable Headend Transmitter 120. The CMTS130A or 130B may be located at a local office of a cable televisioncompany or at another location within a CM communication system. In thefollowing description, the CMTS 130A or 130B is used for illustration;yet, the same functionality and capability as described for the CMTS130A may equally apply to embodiments that alternatively employ the CMTS130B. The cable headend transmitter 120 also provides a number ofservices including those of audio, video, local access channels, as wellas any other service known in the art of cable systems using its CableSystem Broadcasting 134 components. The CMTS 130A provides data servicesto the CMs 111, 115, . . . , 121 that may include Internet access, WideArea Network (WAN) access, and access to other networks to which theCMTS 130A is communicatively coupled. The operation of a CMTS 130A, atthe cable-provider's head-end, may be viewed as providing analogousfunctions provided by a digital subscriber line access multiplexor(DSLAM) within a digital subscriber line (DSL) system. As an example,the CMTS 130A is able to service as many as 1,000 users on a single 6MHz channel. Since a single channel is capable of 30-40 megabits persecond of total throughput, this means that users may see far betterperformance than is available with standard dial-up modems.

[0034] The CMTS 130 takes the traffic coming in from a group ofcustomers on a single channel and routes it to an Internet ServiceProvider (ISP) for connection to the Internet, as shown via the Internetaccess. At the head-end, the cable providers will have, or lease spacefor a third-party ISP to have, servers for accounting and logging,dynamic host configuration protocol (DHCP) for assigning andadministering the Internet protocol (IP) addresses of all the cablesystem's users (CMs 111, 115, . . . , 121), and typically controlservers for a protocol called Data Over Cable Service InterfaceSpecification (DOCSIS), the major standard used by U.S. cable systems inproviding Internet access to users. The servers may also be controlledfor a protocol called European Data Over Cable Service InterfaceSpecification (EuroDOCSIS), the major standard used by European cablesystems in providing Internet access to users, without departing fromthe scope and spirit of the invention.

[0035] The downstream information flows to all of the connected CMs 111,115, . . . , 121 from the CMTS 130B. The upstream information flows fromthe CMs 111, 115, . . . , 121 to the CMTS 130A. The operation of theCable Modem Communication System 100 may be according to Time DivisionMultiple Access (TDMA) operations, Code Division Multiple Access (CDMA),Frequency Division Multiplexing (FDM) operations, or a combination oftwo or more of these. In this manner data intended for, or received fromindividual users is separated.

[0036] In one particular embodiment described below, the teachings ofthe present invention are performed within a CM communication system 100that supports S-CDMA (Synchronous Code Division Multiple Access)operations. In such a S-CDMA system, the CMs 111-121 and the CMTS 130communicate synchronization information to one another such thatupstream transmissions from the CMs 111-121 are time aligned upon theirreceipt by the CMTS 130A. Synchronization of these transmissions at theCMTS 130A in the S-CDMA communication systems is extremely important.When a number of the CMs all transmit their signals at a same time suchthat these signals are received at the CMTS 130 on the same frequencyand at the same time, they must all be able to be properly de-spread anddecoded for proper signal processing. In order to achieve this goal, fora particular transmission cycle, each of the CMs 111-121 transmits tothe CMTS 130A at a respective transmission time, which will likelydiffer from the transmission times of other CMs. These differingtransmission times are based upon the relative transmission distancebetween the CM and the CMTS 130. These operations are supported by adetermination of the round trip delays (RTPs) between the CMTS 130 andeach supported CM. With these RTPs determined, the CMs may thendetermine at what point to transmit their S-CDMA data so that all CMtransmissions will arrive synchronously at the CMTS 130A.

[0037] The CMTS 130A (or CMTS 130B) supports Preamble Erasure ProcessingFunctionality (PEPF). As will be described further with reference toFIGS. 7-14, operation according to the present invention determineswhether any of a group of preamble symbols of a received preamble isreceived by the CMTS 130A coincidence with receipt of burst (impulse)noise. If so, the preamble symbol is not usable. Thus, further accordingto the present invention, when a determination is made that the preamblesymbol is erroneous (received coincident with impulse/burst noise), theCMTS 130A will extract the preamble symbol from the group of preamblesymbols, yielding a subgroup of valid preamble symbols. Furtheraccording to the present invention, the subgroup of valid preamblesymbols is used to estimate the frequency of the preamble, the phase ofthe preamble, and the gain of the preamble. These results may then beused for input gain settings, frequency correction, and phase correctionof data symbols corresponding to the preamble. Further, the subgroup ofvalid preamble symbols may be employed for equalizer training and otheroperations requiring channel characterization.

[0038]FIG. 2 is a system diagram illustrating in more detail a CMTS 130Aor 130B and a CM 111 of the present invention. The CMTS 130A supportsbi-directional communication between a customer premise equipment (CPE)220 and a network 230 or the Internet 240. The CPE 240 may be a personalcomputer or another device allowing a user to access an externalnetwork. In a typical operation, a CM communication system 200 supportsthe flow of Internet protocol (IP) traffic between the Internet 240 andthe CPE 220 via a CM 111. According to the present invention, the CMTS130A or 130B supports PEPF.

[0039] The WAN 230, and/or the Internet 240, is/are communicativelycoupled to the CMTS 130A via the CMTS-NSI. The CMTS 130A is operable tosupport the external network termination, for one or both of the WAN 230and the Internet 240. The CMTS 130A includes a modulator and ademodulator to support transmitter and receiver functionality to andfrom a CM network segment 199. The receiver functionality within theCMTS 130A is operable to support PEPF functionality 210 for S-CDMAaccording to the invention.

[0040]FIG. 3 is a system diagram illustrating a cellular communicationsystem having a base station receiver 340 that services a reverse linkaccording the invention. A mobile transmitter 310 has a local antenna311. The mobile transmitter 310 may be any number of types oftransmitters including a cellular telephone, a wireless pager unit, amobile computer having transmitter functionality, or any other type ofmobile transmitter. The mobile transmitter 310 transmits a signal, usingits local antenna 311, to a base station receiver 340 via a wirelesscommunication channel. The base station receiver 340 is communicativelycoupled to a receiving wireless tower 349 to be able to receivetransmission from the local antenna 311 of the mobile transmitter 310that have been communicated via the wireless communication channel. Thereceiving wireless tower 349 communicatively couples the received signalto the base station receiver 340.

[0041] The base station receiver 340 supports PEPF functionalityaccording to the present invention, as shown in a functional block 341,on the reverse link received signal. FIG. 3 shows just one of manyembodiments where PEPF functionality performed according to theinvention may be performed to provide for improved operation within acommunication system.

[0042]FIG. 4 is a system diagram illustrating a cellular communicationsystem having a mobile receiver 430 that services a forward linkaccording the invention. FIG. 4 illustrates a reverse transmission ofthe cellular communication system 300 of the FIG. 3. A base stationtransmitter 420 is communicatively coupled to a transmitting wirelesstower 421. The base station transmitter 420, using its transmittingwireless tower 421, transmits a signal to a local antenna 439 via awireless communication channel. The local antenna 439 communicativelycouples to a mobile receiver 430 so that the mobile receiver 430 is ableto receive transmission from the transmitting wireless tower 421 of thebase station transmitter 420 that have been communicated via thewireless communication channel. The local antenna 439 communicativelycouples the received signal to the mobile receiver 430. The mobilereceiver 430 may be any number of types of transmitters including acellular telephone, a wireless pager unit, a mobile computer havingtransmitter functionality, or any other type of mobile transmitter.

[0043] The mobile receiver 430 supports PEPF functionality according tothe invention, as shown in a functional block 4311. FIG. 4 shows justone of many embodiments where the PEPF functionality performed accordingto the invention, may be performed to provide for improved operationwithin a communication system.

[0044]FIG. 5 is a system diagram illustrating a satellite communicationsystem 500 and a fixed wireless communication system, each operatingaccording to the invention. A transmitter 520 communicatively couples toa wired network 510. The wired network 510 includes any number ofnetworks including the Internet, proprietary networks, . . . , and otherwired networks. The transmitter 520 includes a satellite earth station551 that is able to communicate to a satellite 553 via a wirelesscommunication channel. The satellite 553 is able to communicate with areceiver 530. The receiver 530 is also located on the earth. Here, thecommunication to and from the satellite 553 may cooperatively be viewedas being a wireless communication channel, or each of the communicationto and from the satellite 553 may be viewed as being two distinctwireless communication channels.

[0045] For example, the wireless communication “channel” may be viewedas not including multiple wireless hops in one embodiment. In otherembodiments, the satellite 553 receives a signal received from thesatellite earth station 551, amplifies it, and relays it to the receiver530; the receiver 530 may include terrestrial receivers such assatellite receivers, satellite based telephones, . . . , and satellitebased Internet receivers, among other receiver types. In the case wherethe satellite 553 receives a signal received from the satellite earthstation 551, amplifies it, and relays it, the satellite 553 may beviewed as being a “transponder.” In addition, other satellites may exist(and operate in conjunction with the satellite 553) that perform bothreceiver and transmitter operations. In this case, each leg of anup-down transmission via the wireless communication channel would beconsidered separately. A wireless communication channel between thesatellite 553 and a fixed earth station would likely be less timevarying than the wireless communication channel between the satellite553 and a mobile station.

[0046] The satellite 553 communicates with the receiver 530. Thereceiver 530 may be a mobile unit employing a local antenna 512.Alternatively, the receiver 530 may be a satellite earth station 552that may be communicatively coupled to a wired network in a similarmanner that the satellite earth station 551, within the transmitter 520,communicatively couples to the wired network 510. In both embodiments,the receiver 530 supports PEPF functionality according to the presentinvention, as shown in a functional block 531.

[0047] As is also illustrated in FIG. 5, a fixed wireless systemincludes a transmitting wireless tower 521 that provides network accessto a plurality of wireless receivers 530. The fixed wireless systemsupports PEPF functionality according to the present invention. Thefixed wireless system may also provide broadcast services similarto/same as the services described with reference to FIG. 1.

[0048]FIG. 6 is a block diagram illustrating a CMTS 130A (or 130B) builtaccording to the present invention. The CMTS system 600 includes a CMTSmedium access controller (MAC) 630 that operates with a number of otherdevices to perform communication from one or more CMs to a WAN 680. TheCMTS MAC 630 provides hardware support for MAC-layer per-packetfunctions including fragmentation, concatenation, and payload headersuppression that all are able to offload the processing required by asystem central processing unit (CPU) 672. This will provide for higheroverall system performance. In addition, the CMTS MAC 630 is able toprovide support for carrier class redundancy via timestampsynchronization across a number of receivers, shown as a receiver 611, areceiver 612, and a receiver 613 that are each operable to receiveupstream analog inputs. In certain embodiments, each of the receivers611, 612, and 613 are dual universal advanced TDMA/CDMA (Time DivisionMultiple Access/Code Division Multiple Access) PHY-layer burstreceivers. That is to say, each of the receivers 611, 612, and 613includes at least one TDMA receive channel and at least one CDMA receivechannel; in this case, each of the receivers 611, 612, and 613 may beviewed as being multi-channel receivers. In other embodiments, thereceivers 611, 612, and 613 includes only CDMA receive channels.

[0049] In addition, the CMTS MAC 630 may be operated remotely with arouting/classification engine 679 that is located externally to the CMTSMAC 630 for distributed CMTS applications including mini fiber nodeapplications. Moreover, a Standard Programming Interface (SPI) masterport may be employed to control the interface to the receivers 611, 612,and 613 as well as to a downstream modulator 620.

[0050] The CMTS MAC 630 may be viewed as being a highly integrated CMTSMAC integrated circuit (IC) for use within the various DOCSIS andadvanced TDMA/CDMA physical layer (PHY-layer) CMTS products. The CMTSMAC 630 employs sophisticated hardware engines for upstream anddownstream paths. The upstream processor design is segmented and usestwo banks of Synchronous Dynamic Random Access Memory (SDRAM) tominimize latency on internal buses. The two banks of SDRAM used by theupstream processor are shown as upstream SDRAM 675 (operable to supportkeys and reassembly) and SDRAM 676 (operable to support Packaging,Handling, and Storage (PHS) and output queues). The upstream processorperforms Data Encryption Standard (DES) decryption, fragment reassembly,de-concatenation, payload packet expansion, packet acceleration,upstream Management Information Base (MIB) statistic gathering, andpriority queuing for the resultant packets. Each output queue can beindependently configured to output packets to either a Personal ComputerInterface (PCI) or a Gigabit Media Independent Interface (GMII). DOCSISMAC management messages and bandwidth requests are extracted and queuedseparately from data packets so that they are readily available to thesystem controller.

[0051] The downstream processor accepts packets from priority queues andperforms payload header suppression, DOCSIS header creation, DESencryption, Cyclic Redundancy Check (CRC) and Header Check Sequence (ofthe DOCSIS specification), Moving Pictures Experts Group (MPEG)encapsulation and multiplexing, and timestamp generation on the in-banddata. The CMTS MAC 630 includes an out-of-band generator and CDMAPHY-layer (and/or TDMA PHY-layer) interface so that it may communicatewith a CM device's out-of-band receiver for control of power managementfunctions. The downstream processor will also use SDRAM 677 (operable tosupport PHS and output queues). The CMTS MAC 630 may be configured andmanaged externally via a PCI interface and a PCI bus 671.

[0052] Each of the receivers 611, 612, and 613 is operable to supportPEPF functionality for CDMA. For example, the receiver 611 is operableto support PEPF functionality for CDMA, the receiver 612 is operable tosupport PEPF functionality for CDMA, and the receiver 613 is operable tosupport PEPF functionality for CDMA. FIG. 6 shows yet another embodimentin which PEPF functionality for CDMA may be performed according to theinvention. Any of the functionality and operations described in theother embodiments may be performed within the context of the CMTS system600 without departing from the scope and spirit of the invention.

[0053]FIG. 7 is a graph illustrating preamble and data symbols receivedand operated upon by a CMTS (or another) receiver according to thepresent invention. To illustrate the ideas of the present inventionwithout loss of generality, the discussion of the present invention ismade with reference to S-CDMA systems. In S-CDMA systems, the preambleand data signals transmitted by different users, i.e., CMs, aretransmitted on different spreading codes. All signals are synchronouslyadded on the channel supported by the CM network segment 199. Burst(impulse) noise is also added to the combined signal. The non-CDMA casecould be also treated within the same framework, as a single user signalwith no spreading, which is corrupted by impulse noise.

[0054]FIG. 7 shows a general S-CDMA signal structure after despreading.The horizontal axis of the rectangular frame structure represents time(in symbol intervals) and the vertical axis represents differentspreading codes. Here many user signals could be transmitted at the sametime but on different spreading codes. Nine symbol times, n=1, . . . ,9, are illustrated in FIG. 7. As is shown, some spreading codes carryingpreamble symbols while other spreading codes carry data. FIG. 7 alsoillustrates the corruptive effect of impulse noise on S-CDMA signalsafter despreading (during symbol times n=7 and n=8). Here we note thatthe impulse noise is added to the signal in the time domain, thus itaffects only a few columns of the S-CDMA frame structure.

[0055] According to the present invention, it is first determined whichof the symbols of the preamble have been corrupted by impulse noise.With this information determined, the following benefits may berealized:

[0056] 1. Using corrupted preamble symbols in the columns corrupted bystrong impulse noise in the estimation of user parameters such as gain,phase, and frequency offsets is avoided. Thus, only clean preamblesymbols in the estimation process are employed. If corrupted preamblesymbols are blindly included in the estimation process, the systemthroughput could be reduced significantly.

[0057] 2. Forward error correction (FEC) decoders, such as Viterbi andReed-Solomon decoders, may exploit knowing which data are corrupted byimpulse noise to enhance their data error correction capability.

[0058]FIG. 8 is a logic diagram illustrating operation of the presentinvention in operating upon a preamble to detect burst (impulse) noiseand in operating upon the preamble based upon the determination. Theoperations of FIG. 8 are performed within a digital communicationreceiver, such as those illustrated with reference to FIGS. 1-6. Variousstructures may be employed to perform the operations described herein.Examples of such structures include both dedicated signal pathstructures and non-dedicated signal path processors. The operations ofFIG. 8 would typically be performed in a baseband processor afterreceived signals have been converted to baseband. The operations of thepresent invention are not limited to a particular structure. Operationcommences with the receipt of one or more preamble sequences. As wasdescribed with reference to FIG. 7, in a S-CDMA system, a CMTS willreceive a number of uplink signals from a plurality of serviced CMs.These signals arrive spread and synchronized in time. One or more ofthese signals is a preamble sequence that includes a group of preamblesymbols. Thus, as a first operation of FIG. 8, the preamble sequence(s)are received (step 802). In a S-CDMA system or another CDMA system, thepreamble sequences may have been despread when the operation of step 802is performed. With further description of FIG. 8, each received preamblesequence is denoted as: {x(1),x(2), . . . ,x(L)}, where L is thepreamble length. With continued discussion of FIG. 8 and subsequentFIGs., it is assumed that the preamble extends only over one code.

[0059] Next, the received preamble symbols are divided by known preamblesymbols to obtain the sequence {z(k)=x(k)/p(k)}, where {p(k)} is theknown preamble sequence (step 804). The sequence {z(k)}could be averagedvertically if it extends over multiple codes in the SCDMA case. Next, again differential sequence α(k) is obtained (step 806). In oneembodiment, the gain differential sequence is determined according toEq. (1)

Δα(k)=|z(k+1)|−|z(k)|, k=1 . . . L−1

Δα(L)=|z(1)|−|z(L)|

α(k)=|Δα(k)|  Eq. (1)

[0060] Next, a phase differential sequence d(k) is obtained (step 808).In one embodiment, the phase differential sequence d(k) is determinedaccording to Eq. (2):

Δφ(k)=angle[z(k+1)z*(k)], k=1 . . . L−1

ω_(coarse)=mean[Δφ(k)], k=2 . . . L−2

ΔΦ(L)=angle[z(1)z*(L)]+(L−2)ω_(coarse)

d(k)=|ΔΦ(k)−ω_(coarse)  Eq. (2)

[0061] Impulse noise is then detected from one or both of the gaindifferential sequence and the phase differential sequence (step 810). Inone operation, the detection of impulse noise is based on comparing thetwo sequences α(k) and d(k) to programmable thresholds that are computedbased on known information of the SNR of the channel (noise variance).In particular, these determinations may be determined according to Eq.(3)

if[d(k)>th&d(k−1)>th]OR[α(k)>th&α(k−1)>th]

erase_flag(k)=1  Eq. (3)

[0062] The erase_flag(k) for each symbol period is then used todetermine which symbols of the preamble sequence are good (step 812).The same procedure is then repeated for different user preamblesequences and the obtained detection results could be combined to obtaina higher probability of accurate detection. The good symbols of thepreamble sequence may be considered a subgroup of symbols. This subgroupof good symbols is then combined to produce a composite result (step814). This composite result may then be used to estimate the frequencyof the preamble, the phase of the preamble, and the gain of thepreamble. These results may then be used for input gain settings,frequency correction, and phase correction of data symbols correspondingto the preamble. Further, the subgroup of valid preamble symbols may beemployed for equalizer training and other operations requiring channelcharacterization.

[0063]FIG. 9 is a graph illustrating preamble gain of a plurality ofpreamble symbols as determined according to the present invention. Asshown in FIG. 9, the preamble gain for symbol periods n=1 through n=9 isconsidered. As noted, the symbol of symbol time n=8 has a largerpreamble gain than do the other symbols.

[0064]FIG. 10 is a graph illustrating delta preamble gain of a pluralityof preamble symbols and related error detection as determined accordingto the present invention. When the delta preamble gain for the preamblesequence of FIG. 9 is considered, the resultant for symbol times n=7 andn=8 violate selected thresholds. Such result provides a first indicationthat the preamble symbols corresponding to symbol times n=7 and n=8coexist with impulse noise. These indications compare favorably to theposition of the impulse noise as illustrated in FIG. 7. The symbolscorresponding to symbol positions n=7 and n=8 are erased fromconsideration in further processing.

[0065]FIG. 11 is a graph illustrating preamble phase of a plurality ofpreamble symbols as determined according to the present invention. Asillustrated, the phase of the preamble sequence is increasing with timeand most of the preamble phase of the symbols falls along a straightline. However, the preamble symbols corresponding to symbol times n=7and n=8 do not correspond to this straight line.

[0066]FIG. 12 is a graph illustrating delta preamble phase of aplurality of preamble symbols and related error detection as determinedaccording to the present invention. As is shown, the delta preamblephase methodology of the method of the present invention also determinesthat the symbols corresponding to symbol positions n=7 and n=8 are badand should be erased from consideration in further processing.

[0067]FIG. 13 is a graph illustrating erasure flags corresponding to aplurality of preamble symbols as determined according to the presentinvention. The results of the operations of FIG. 8 are summarized inFIG. 13 to indicate that the preamble symbols corresponding to symboltimes n=7 and n=8 should be removed from further consideration insubsequent processing. The operations of FIG. 14 will describe how theseoperations are accomplished.

[0068] In a S-CDMA system, the procedure described with reference toFIGS. 8-13 may be repeated for different user preamble sequences and theobtained detection results may be combined to obtain a higherprobability of accurate detection. The detection procedure may limitedto the detection of bad symbols based upon gain only or phase only. Thedetection process could also be limited to a given number of users basedon the available processing power in the system.

[0069]FIG. 14 is a logic diagram illustrating operation of the presentinvention in operating upon a preamble to detect burst (impulse) noise,to erase preamble symbols, to combine good preamble symbols, and todetermine frequency, phase, and gain estimates of the preamble. Thus, asa first operation of FIG. 14, the preamble sequence(s) are received(step 1402). Next, bad symbols in the received preamble sequence(s) aredetermined (step 806). Once the bad symbols in each of received preamblesequence(s) are determined, bad symbols are masked and correctionfactors are also determined for the masked symbols (step 1406). Thecorrection factors are applied to the erased/bad symbols in thesubgroup(s) of symbols.

[0070] The resulting sequence is divided into subgroups and the goodelements in each subgroup are combined (step 1408). In this operation,good symbols are not erased. Subgroup symbol combining may be performedby averaging according to Eq. (4): $\begin{matrix}{{z_{s}(n)} = {\sum\limits_{n = {{{({i - 1})}L_{1}} + 1}}^{{nL}_{1}}{{z(k)} \cdot {\overset{\_}{e}(k)}}}} & \text{Eq.~~(4)}\end{matrix}$

[0071] For each subgroup, the following quality parameters (andcorrection parameters) are determined (step 1410): $\begin{matrix}{{{{d_{L1}(k)} = \frac{k - 1 - {\left( {L_{1} - 1} \right)/2}}{L_{1} - 1}},{k = {1\quad \ldots \quad L_{1}}}}{{c_{s}(n)} = {{\sum\limits_{n = {{{({i - 1})}L_{1}} + 1}}^{{nL}_{1}}{{d_{L1}(k)} \cdot {e(k)}}} + {\left( {n - 1} \right)L_{1}}}}{{e_{s}(n)} = {\sum\limits_{k = {{{({n - 1})}L_{1}} + 1}}^{{nL}_{1}}{e(k)}}}} & \text{Eq.~~(5)}\end{matrix}$

[0072] Here, d_(L1)(k)·ω is the phase difference caused by dropping thek^(th) sample, c_(s)(n)·ω is the total phase difference caused to thenth sub-group, and e_(s)(n) is the number of bad symbols in the n^(th)sub-group. Then, bad subgroup(s) are discarded (step 1412) such thatonly good subgroups are kept. The operations of FIG. 1412 may beperformed according to Eq. (6) as follows:

z _(g) =[ ];c _(g)=[ ];

if e _(s)(n)<L ₁ :z _(g) =[z _(g) ,z _(s)(n)];c _(g) =[c _(g) ,c_(s)(n)];

n=1,2, . . . ,L/L ₁  Eq. (6)

[0073] Then, the phase difference between subgroups is are computed (atstep 1414 if more than one subgroup is considered) by: $\begin{matrix}{{{{\Delta\varphi}(m)} = \frac{{angle}\left\lfloor {{z_{g}(m)} \cdot {z_{g}^{*}\left( {m + 1} \right)}} \right\rfloor}{{c_{g}\left( {m + 1} \right)} - {c_{g}(m)}}},{m = 1},2,\ldots \quad,{{{length}\left( z_{g} \right)} - 1}} & \text{Eq.~~(7)}\end{matrix}$

[0074] A frequency estimate for the preamble is computed then computed(step 1416). This frequency estimate may be determined according to Eq.(8) as: $\begin{matrix}{\hat{\omega} = {\frac{1}{I} \cdot {{mean}\left\lbrack {{\Delta\varphi}(m)} \right\rbrack}}} & \text{Eq.~~(8)}\end{matrix}$

[0075] where I is the interleaving depth of the preamble sequence.Similarly, phase estimates (step 1418) and gain estimates (step 1420)are obtained. The phase and frequency estimates may be determinedaccording to: $\begin{matrix}{{{{d_{L}(k)} = \frac{k - 1 - {\left( {L - 1} \right)/2}}{L - 1}},{k = {1\quad \ldots \quad L}}}{{c_{\varphi} = {{\left( {L - 1} \right)/2} + {\sum\limits_{k = 1}^{L}{{d_{L}(k)} \cdot {e(k)}}}}},{c_{\alpha} = \frac{1}{L - {\sum\limits_{k = 1}^{L}{e(k)}}}}}} & \text{Eq.~~(9)} \\{\hat{\varphi} = {{{angle}\left\lbrack {\sum\limits_{k = 1}^{L}{{z(k)} \cdot {\overset{\_}{e}(k)}}} \right\rbrack} - {c_{\varphi} \cdot \hat{\omega}}}} & \text{Eq.~~(10)} \\{\hat{\alpha} = {{{abs}\left\lbrack {\sum\limits_{k = 1}^{L}{{z(k)} \cdot {\overset{\_}{e}(k)}}} \right\rbrack} \cdot c_{\alpha}}} & \text{Eq.~~(11)}\end{matrix}$

[0076] With estimates for the carrier frequency, phase, and gain offsetsin hand, the whole despread data could be corrected for these offsets(step 1422). The corrected data using the initial offset estimates issliced. Each despread data symbol is divided by the corresponding sliceddata decision. The obtained sequence is then averaged across differentcodes to obtain a less noisy sequence, which is then used to estimatethe carrier frequency, phase, and gain offsets again. The procedure canbe repeated (iterated) to obtain a more accurate carrier offsetestimates.

[0077]FIG. 15 is a graph illustrating how system performance improveswhen operating according to the present invention. Operation accordingto the present invention occurs in FIG. 15 with a baud rate of 5.12 MHz,an impulse noise duration of 10 micro sec. (One hit/frame) with K=32,L=32, a 64 QAM constellation and a SNR of 35 dB.

[0078]FIG. 16 is a block diagram illustrating a communication deviceconstructed according to the present invention. The communication deviceincludes a communication device front end 1602 and a communicationreceiver 1604. The communication device front end 1602 receives anincoming analog signal and that processes the incoming analog signal toproduce a preamble sequence. The communication receiver 1604 operablycouples to the communication device front end and performs a pluralityof operations to detect impulse noise in the preamble sequence. As afirst operation, the communication receiver receives the preamblesequence that includes a plurality of preamble symbols. Thecommunication receiver then divides the plurality of preamble symbols byat least one known preamble symbol to produce a plurality of preamblegains and/or a plurality of preamble phases corresponding to theplurality of preamble symbols.

[0079] Finally, the communication receiver determines, based upon theplurality of preamble gains and/or the plurality of preamble phases,that at least one preamble symbol has been adversely affected by impulsenoise. The communication receiver 1604 may include a dedicatedcomponent, e.g., a preamble processor 1606 that performs the operationsof the present invention. Alternately, a general processing component,e.g., a Digital Signal Processor, host processor, or another processormay perform operations according to the present invention. In eithercase, the processing of incoming signals may be performed via adedicated or non-dedicated signal path, depending upon the embodiment.

[0080] In view of the above detailed description of the invention andassociated drawings, other modifications and variations will now becomeapparent. It should also be apparent that such other modifications andvariations may be effected without departing from the spirit and scopeof the invention.

1. A method for processing a preamble in the presence of impulse noise,the method comprising: receiving a preamble sequence that includes aplurality of preamble symbols; determining that at least one preamblesymbol of the plurality of preamble symbols has been adversely affectedby impulse noise; masking the at least one preamble symbol of theplurality of preamble symbols to produce a subgroup of preamble symbols;and based upon the subgroup of preamble symbols, determining at leastone of a frequency estimate for the subgroup of preamble symbols, aphase estimate for the subgroup of preamble symbols, and a gain estimatefor the subgroup of preamble symbols.
 2. The method of claim 1, furthercomprising: determining a correction factor for the subgroup of preamblesymbols; and applying the correction factor to the subgroup of preamblesymbols to produce a corrected subgroup of preamble symbols.
 3. Themethod of claim 2, wherein the correction factor corrects at least oneof frequency offset, phase offset, and gain offset of the subgroup ofpreamble symbols.
 4. The method of claim 2, further comprisingestimating channel parameters based upon the corrected subgroup ofpreamble symbols.
 5. The method of claim 2, further comprising applyingthe correction factor for the subgroup of preamble symbols to aplurality of data symbols that correspond to the plurality of preamblesymbols.
 6. The method of claim 1, wherein determining that at least onepreamble symbol of the plurality of preamble symbols has been adverselyaffected by impulse noise comprises: dividing the plurality of preamblesymbols by at least one known preamble symbol to produce a plurality ofpreamble gains corresponding to the plurality of preamble symbols; anddetermining, based upon the plurality of preamble gains, that at leastone preamble symbol has been adversely affected by impulse noise.
 7. Themethod of claim 6, wherein determining, based upon the plurality ofpreamble gains, that at least one preamble symbol has been adverselyaffected by impulse noise comprises: from the plurality of preamblegains, determining a gain differential sequence that includes aplurality of gain differential values; and for each gain differentialvalue that exceeds a gain differential threshold, determining that acorresponding preamble symbol has been adversely affected by impulsenoise.
 8. The method of claim 1, wherein determining that at least onepreamble symbol of the plurality of preamble symbols has been adverselyaffected by impulse noise comprises: dividing the plurality of preamblesymbols by at least one known preamble symbol to produce a plurality ofpreamble phases corresponding to the plurality of preamble symbols; anddetermining, based upon the plurality of preamble phases, that at leastone preamble symbol has been adversely affected by impulse noise.
 9. Themethod of claim 8, wherein determining, based upon the plurality ofpreamble phases, that at least one preamble symbol has been adverselyaffected by impulse noise comprises: from the plurality of preamblephases, determining a phase differential sequence that includes aplurality of phase differential values; and for each phase differentialvalue that exceeds a phase differential threshold, determining that acorresponding preamble symbol has been adversely affected by impulsenoise.
 10. The method of claim 1, further comprising combining thesubgroup of preamble symbols to produce a composite result.
 11. Themethod of claim 1, wherein the preamble sequence is received via a wiredcommunication link.
 12. The method of claim 1, wherein the preamblesequence is received via a wireless communication link.
 13. The methodof claim 1, wherein the preamble sequence is received and processed by acable modem communication system receiver.
 14. The method of claim 13,wherein the preamble sequence is received and processed by a Cable ModemTermination System.
 15. A communication device comprising: acommunication device front end that receives an incoming analog signaland that processes the incoming analog signal to produce a preamblesequence; a communication receiver operably coupled to the communicationdevice front end that performs a plurality of operations to process thepreamble sequence, wherein the communication receiver: receives thepreamble sequence that includes a plurality of preamble symbols;determines that at least one preamble symbol of the plurality ofpreamble symbols has been adversely affected by impulse noise; masks theat least one preamble symbol of the plurality of preamble symbols toproduce a subgroup of preamble symbols; and based upon the subgroup ofpreamble symbols, determines at least one of a frequency estimate forthe subgroup of preamble symbols, a phase estimate for the subgroup ofpreamble symbols, and a gain estimate for the subgroup of preamblesymbols.
 16. The communication device of claim 15, wherein thecommunication receiver further: determines a correction factor for thesubgroup of preamble symbols; and applies the correction factor to thesubgroup of preamble symbols to produce a corrected subgroup of preamblesymbols.
 17. The communication device of claim 16, wherein thecommunication receiver applies the correction factor to the preamblesymbols in order to correct at least one of frequency offset, phaseoffset, and gain offset.
 18. The communication device of claim 16,wherein the communication receiver further estimates channel parametersbased upon the corrected subgroup of preamble symbols.
 19. Thecommunication device of claim 16, wherein the communication receiverfurther applies the correction factor for the subgroup of preamblesymbols to a plurality of data symbols that correspond to the pluralityof preamble symbols.
 20. The communication device of claim 15, whereinthe communication receiver determines that at least one preamble symbolof the plurality of preamble symbols has been adversely affected byimpulse noise by: dividing the plurality of preamble symbols by at leastone known preamble symbol to produce a plurality of preamble gainscorresponding to the plurality of preamble symbols; and determining,based upon the plurality of preamble gains, that at least one preamblesymbol has been adversely affected by impulse noise.
 21. Thecommunication device of claim 20, wherein the communication receiverdetermines, based upon the plurality of preamble gains, that at leastone preamble symbol has been adversely affected by impulse noise by:from the plurality of preamble gains, determining a gain differentialsequence that includes a plurality of gain differential values; and foreach gain differential value that exceeds a gain differential threshold,determining that a corresponding preamble symbol has been adverselyaffected by impulse noise.
 22. The communication device of claim 15,wherein the communication receiver determines that at least one preamblesymbol of the plurality of preamble symbols has been adversely affectedby impulse noise by: dividing the plurality of preamble symbols by atleast one known preamble symbol to produce a plurality of preamblephases corresponding to the plurality of preamble symbols; anddetermining, based upon the plurality of preamble phases, that at leastone preamble symbol has been adversely affected by impulse noise. 23.The communication device of claim 22, wherein the communication receiverdetermines, based upon the plurality of preamble phases, that at leastone preamble symbol has been adversely affected by impulse noise by:from the plurality of preamble phases, determining a phase differentialsequence that includes a plurality of phase differential values; and foreach phase differential value that exceeds a phase differentialthreshold, determining that a corresponding preamble symbol has beenadversely affected by impulse noise.
 24. The communication device ofclaim 15, wherein the communication receiver further combines thesubgroup of preamble symbols to produce a composite result.
 25. Thecommunication device of claim 15, wherein the communication device frontend services a wired communication link.
 26. The communication device ofclaim 15, wherein the communication device front end services a wirelesscommunication link.
 27. The communication device of claim 15, whereinthe communication device front end services a cable modem communicationsystem receiver.
 28. The communication device of claim 27, wherein thecommunication device is a Cable Modem Termination System.
 29. Acommunication receiver comprising: means for receiving a preamblesequence that includes a plurality of preamble symbols; means fordetermining that at least one preamble symbol of the plurality ofpreamble symbols has been adversely affected by impulse noise; means formasking the at least one preamble symbol of the plurality of preamblesymbols to produce a subgroup of preamble symbols; and means for basedupon the subgroup of preamble symbols, determining at least one of afrequency estimate for the subgroup of preamble symbols, a phaseestimate for the subgroup of preamble symbols, and a gain estimate forthe subgroup of preamble symbols.